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Lifecycle hooks & Finalizers
The ArangoDB operator expects full control of the Pods and PersistentVolumeClaims it creates.
Therefore it takes measures to prevent the removal of those resources
until it is safe to do so.
To achieve this, the server containers in the Pods have
a preStop hook configured and finalizers are added to the Pods
and PersistentVolumeClaims.
The preStop hook executes a binary that waits until all finalizers of
the current pod have been removed.
Until this preStop hook terminates, Kubernetes will not send a TERM signal
to the processes inside the container, which ensures that the server remains running
until it is safe to stop them.
The operator performs all actions needed when a delete of a Pod or
PersistentVolumeClaims has been triggered.
E.g. for a dbserver it cleans out the server if the Pod and PersistentVolumeClaim are being deleted.
Lifecycle init-container
Because the binary that is called in the preStop hook is not part of a standard
ArangoDB docker image, it has to be brought into the filesystem of a Pod.
This is done by an initial container that copies the binary to an emptyDir volume that
is shared between the init-container and the server container.
Finalizers
The ArangoDB operators adds the following finalizers to Pods.
dbserver.database.arangodb.com/drain: Added to DBServers, removed only when the dbserver can be restarted or is completely drainedagent.database.arangodb.com/agency-serving: Added to Agents, removed only when enough agents are left to keep the agency serving
The ArangoDB operators adds the following finalizers to PersistentVolumeClaims.
pvc.database.arangodb.com/member-exists: removed only when its member exists no longer exists or can be safely rebuild